138 Modern Food Microbiology
found in fruits and bringing about fermentation with the production of alcohol and carbon dioxide.
Due to their generally faster growth rate than molds, they generally precede the latter organisms
in the spoilage process of fruits in certain circumstances. It is not clear whether some molds are
dependent on the initial action of yeasts in the process of fruit and vegetable spoilage. The utilization
or destruction of the high-molecular-weight constituents of fruits is brought about more by molds
than yeasts. Many molds are capable of utilizing alcohols as sources of energy, and when these and
other simple compounds have been depleted, these organisms proceed to destroy the remaining parts
of fruits, such as the structural polysaccharides and rinds. Fire blight of apple and pear trees is caused
by Erwinia amylovora. One method that has been studied to control the fire blight organism involves
the use of Pantoea agglomerans and P. dispersa as biological control agents since they do not harm
apple or pear trees, yet they prevent the pathogen’s invasion. In vitro studies have revealed the P.
agglomerans inhibition to be an antibiotic complex designated pantocin A and pantocin B.
61 These
inhibitors appear to be effective against other Gram-negative bacteria.
FRESH-CUT PRODUCE
The production of precut packaged fruit and vegetable salads (minimally processed) has led to an explosion in the sale and consumption of these commodities during the past decade, and this trend shows
signs of continuing. In essence, salad vegetables such as lettuce and carrots, and fruits such as cantaloupes and watermelons are cut, sliced, and packaged in see-through containers that are stored at chill
temperatures, such that they are ready-to-use (RTU) upon purchase. If packaged in high-oxygen permeable films, the primary concerns are product quality and enzymatic browning in the case of light-colored
products. However, when low-O 2 permeable packaging is used with long-term storage, the possibility
exists for the growth of microbial pathogens such as C. botulinum and L. monocytogenes. This concern
has led to numerous studies on the safety of the final RTU produce, and some of these are summarized
below. Since modified atmosphere/vacuum packaging is often used for these products, some relevant
information can be found in Chapter 14. More extensive information can be found in reference 14.
Microbial Load
Overall, RTU (ready-to-use, ready-to-eat) produce is by no means microbe-free. In their preparation,
intact vegetables are washed, typically with water that contains chlorine from 50 to 200 ppm, followed
by cutting and packaging. While washing reduces microbial numbers, the cutting operation has the
potential to recontaminate. Also, the freshcut vegetables provide a higher level of moisture, more
simple nutrients, and a higher surface area, all of which make the RTU product more susceptible to
microbial growth than the original.
The APCs of eight RTU vegetables in Ontario, Canada, recorded on day 0 and day 4 after storage at
4
◦ C are presented in Table 6–10.
33 It can be seen that the initial numbers ranged from 4.82 log 10 /g to
near 6.0 log 10 /g on day 0, but after a 4-day storage, they ranged from 5.45 to >7.0 log 10 /g. In an earlier
study, the APC of RTU vegetables at harvest was around 10
5
− 10
8 /g, and after storage at 7
◦ C, the
APC at time of sell-by date +1 day for 12 vegetables ranged between 7.7 and 9.0 log 10 /g, a time when
all products were organoleptically acceptable.
10 In the first study,
33 coliforms ranged from 5.1 to 7.2
log 10 /g, but no type 1 E. coli strains were found. The most predominant organisms were Pseudomonas
and Pantoea. In a study
4 of the types of organisms on RTU, spinach that was stored at 10
◦ C for 12
days, mesophiles ranged between 10
7 and 10
10 /g, psychrotrophs and pseudomonads between 10
6 and
found in fruits and bringing about fermentation with the production of alcohol and carbon dioxide.
Due to their generally faster growth rate than molds, they generally precede the latter organisms
in the spoilage process of fruits in certain circumstances. It is not clear whether some molds are
dependent on the initial action of yeasts in the process of fruit and vegetable spoilage. The utilization
or destruction of the high-molecular-weight constituents of fruits is brought about more by molds
than yeasts. Many molds are capable of utilizing alcohols as sources of energy, and when these and
other simple compounds have been depleted, these organisms proceed to destroy the remaining parts
of fruits, such as the structural polysaccharides and rinds. Fire blight of apple and pear trees is caused
by Erwinia amylovora. One method that has been studied to control the fire blight organism involves
the use of Pantoea agglomerans and P. dispersa as biological control agents since they do not harm
apple or pear trees, yet they prevent the pathogen’s invasion. In vitro studies have revealed the P.
agglomerans inhibition to be an antibiotic complex designated pantocin A and pantocin B.
61 These
inhibitors appear to be effective against other Gram-negative bacteria.
FRESH-CUT PRODUCE
The production of precut packaged fruit and vegetable salads (minimally processed) has led to an explosion in the sale and consumption of these commodities during the past decade, and this trend shows
signs of continuing. In essence, salad vegetables such as lettuce and carrots, and fruits such as cantaloupes and watermelons are cut, sliced, and packaged in see-through containers that are stored at chill
temperatures, such that they are ready-to-use (RTU) upon purchase. If packaged in high-oxygen permeable films, the primary concerns are product quality and enzymatic browning in the case of light-colored
products. However, when low-O 2 permeable packaging is used with long-term storage, the possibility
exists for the growth of microbial pathogens such as C. botulinum and L. monocytogenes. This concern
has led to numerous studies on the safety of the final RTU produce, and some of these are summarized
below. Since modified atmosphere/vacuum packaging is often used for these products, some relevant
information can be found in Chapter 14. More extensive information can be found in reference 14.
Microbial Load
Overall, RTU (ready-to-use, ready-to-eat) produce is by no means microbe-free. In their preparation,
intact vegetables are washed, typically with water that contains chlorine from 50 to 200 ppm, followed
by cutting and packaging. While washing reduces microbial numbers, the cutting operation has the
potential to recontaminate. Also, the freshcut vegetables provide a higher level of moisture, more
simple nutrients, and a higher surface area, all of which make the RTU product more susceptible to
microbial growth than the original.
The APCs of eight RTU vegetables in Ontario, Canada, recorded on day 0 and day 4 after storage at
4
◦ C are presented in Table 6–10.
33 It can be seen that the initial numbers ranged from 4.82 log 10 /g to
near 6.0 log 10 /g on day 0, but after a 4-day storage, they ranged from 5.45 to >7.0 log 10 /g. In an earlier
study, the APC of RTU vegetables at harvest was around 10
5
− 10
8 /g, and after storage at 7
◦ C, the
APC at time of sell-by date +1 day for 12 vegetables ranged between 7.7 and 9.0 log 10 /g, a time when
all products were organoleptically acceptable.
10 In the first study,
33 coliforms ranged from 5.1 to 7.2
log 10 /g, but no type 1 E. coli strains were found. The most predominant organisms were Pseudomonas
and Pantoea. In a study
4 of the types of organisms on RTU, spinach that was stored at 10
◦ C for 12
days, mesophiles ranged between 10
7 and 10
10 /g, psychrotrophs and pseudomonads between 10
6 and